A conventional sensor output processing device has a signal limiter which limits the output signal of the processing device to be within a predetermined signal range, for instance to the range from a low limit level (0.5 volts) to a high limit level (4.5 volts), although the device is operable to produce the output signal from the low side voltage (0 volt) to the high side voltage (5 volts) of a power source. That is, the output signal is limited to be different from the power source voltages (0 volts and 5 volts).
It is proposed to provide this processing device with a self-diagnosis function which continues to limit the output signal to the predetermined limit, 0.5 or 4.5 volts, when any abnormality in a sensor or its electronic signal processing circuit is detected. If the output signal from the signal processing circuit continues to be at the limit level, an external device, which receives the output signal thus continuously limited, recognizes that the sensor or the processing circuit has an abnormality. In this instance, however, it is difficult for the external circuit to determine whether the output signal is limited to the limit level in the normal operation or the output signal is maintained at the limit level due to abnormality in the sensor. Therefore, it is desired to provide a predetermined signal margin ΔV between the limit level of the output signal and a diagnosis signal indicative of abnormality. This signal margin will narrow the signal range in which the output signal is allowed to vary.